Monday, May 7, 2018

Thermocouples are simple devices made up of several key components: thermocouple wire, electrical insulation, and the sensing junction. Many thermocouple designs also include a stainless steel sheath that protects the thermocouple from vibration, shock, and corrosion.

A thermocouple has three variations of sensing tip (or junction):

Exposed junction, where the exposed wire tips and welded bead have no covering or protection.

Grounded junction, where the welded bead is in physical contact with the thermocouple's sheath.

Ungrounded junction, where the tip is inside the thermocouple sheath, but is electrical (and somewhat thermally) insulated from the sheath (no sheath contact).

Exposed junction thermocouples respond to temperature change quickly and are less costly, but their signals are susceptible erratic reading caused by induced or conducted electrical noise. Because there is no sheath, they are also prone to mechanical damage and ambient contamination.

Grounded junction thermocouples provide fast response and are mechanically more robust, with a metallic sheath that protects the thermocouple both mechanically and from contaminants. But because their sensing tip is in contact with the external sheath, their signal still can be affected by externally induced or conducted electrical noise.

Ungrounded thermocouples, like grounded, are protected mechanically and from ambient contaminants by their sheath. However, their sensing junctions are kept separate from their metallic sheath, isolating the junction from external electrical interference. This separation does come at a small cost in temperature sensing responsiveness though.

Tuesday, March 27, 2018

The TX200 is a compact, rugged pressure transmitter utilizing ASIC technology to provide optimum sensor signal conditioning and temperature compensation of the sensor output. It is designed for process control industries worldwide and ideally suited for petrochemical and upstream oil and gas applications. The TX200 provides a cost-effective solution to using conventional process transmitters.

The fixed range model TX200B is recommended for use where process pressure is consistent within the range and where physical access to the transmitter is limited or not required.

The field adjustable model TX200A allows access to zero and span the transmitter. The transmitter may be spanned up to 5:1 and for ease of calibration, does not require a calibrated pressure source and can be calibrated in-place.

Both TX200 models feature an all welded, 316 stainless steel hermetically sealed enclosure providing airtight and watertight protection within the harshest environments. A 316 stainless steel, rotatable cover protects product markings and adjustment buttons (TX200A) from the elements and tampering. The TX200 lends itself to control panel mounting or direct process mounting due to its light-weight, cylindrical design.

TX200H Hart Models

The TX200H is a HART Smart pressure transmitter that provides simplified eld adjustment while reliably communicating asset management data utilizing the latest HART 7 specification. A proprietary calibration process insures optimum temperature compensation limiting thermal effects on the sensor output. As with the ASIC TX200, it is suited for process control industries worldwide and provides a cost-effective solution to using conventional HART transmitters.

Wednesday, October 25, 2017

United Electric Controls has a rich history of over 80 years in providing protection for plant assets, people and the environment. Their pressure and temperature instrumentation is designed specifically to meet the rigors of harsh and hazardous alarm and emergency shutdown applications and includes certified safety transmitters per IEC 61508. UE, and Ives Equipment, serves the Chemical & Petrochemical, Power, Water & Wastewater and Oil & Gas industries, as well as many other challenging OEM applications.

Tuesday, October 24, 2017

Not all processes or operations require the use of state of the art technology to get the desired result. Part of good process design is matching up the most appropriate methods and technology to the operation.

One method of changing the state of an electrical switch from open to closed in response to a process temperature change is a bulb and capillary temperature switch. The change in state occurs in the mechanical switch when the temperature of a process control operation crosses a certain threshold. Bulb and capillary switches have the advantage of operating without requiring an excitation voltage, simplifying their use in a given application.

The physical operating principle behind the capillary thermostat relies on the use of a fluid. The fluid inside the thermostat expands or contracts in response to the temperature at the sensing bulb. The change in fluid volume produces a force upon a diaphragm or other mechanical transfer device. The diaphragm is connected to, and changes the status of, an adjoining circuit using a snap action switch.

Because of their simplicity and comparatively modest cost, commercial versions of bulb and capillary switches find application throughout residential and commercial settings. Some common applications include warming ovens, deep fat fryers, and water heaters.

Industrial versions of bulb and capillary switches are fitted with appropriate housings for the installation environment. Housings designed for hazardous areas, drenching or submersion, high dust or high corrosive environments are standardly available. Many switching options exist as well, such as high current ratings, SPDT, DPDT, dual SPDT, adjustable deadbands, and internal or external adjustments.

Operation of the temperature switches is subject to a few limitations. The setpoint is most often fixed, so changing the setpoint accurately requires trial and error or a calibration procedure. The temperature range over which the switches are suitable is comparatively limited, with a matching of the bulb and capillary fluid system to the application temperature range a necessary task in product selection. Within its proper sphere of use, though, bulb and capillary temperature switches offer simple, reliable operation, with little requirement for maintenance.

Wednesday, May 24, 2017

This video below demonstrates how to replace an older on/off mechanical pressure switch and install the UE One Series.

The One Series electronic pressure and temperature transmitter-switches set the standard for smart digital process monitoring. With a fully adjustable set point and deadband and 0.1% repeatability, the One Series performs in a wide variety of applications. Available in Type 4X enclosures approved for intrinsic safety, flameproof and non-incendive area classifications, these hybrid transmitter-switches are designed to provide transmitter, switch and gauge functions all-in-one rugged enclosure that can withstand the rigors of harsh and hazardous environments.

Each One Series model incorporates intelligent self-diagnostics and can report detected faults before they become major safety issues. Plug Port Detection protects against sensor clogging. Nuisance trip filtering reduces false and spurious signals. The ability to capture pressure spikes and valleys provides process information to aid in the commissioning and debugging process.

The normal status of a switch can be a confusing aspect of understanding the function of connected electrical and logic components in a process control application. The misunderstanding stems from the ambiguity of the word normal. Typically, electrical switch contacts are classified as being normally-open or normally-closed, referring to the open or closed status of the contacts under normal conditions. The key in understanding the normal state of a switch contact requires one to dissociate from their thinking, the concept or definition of normal used in everyday conversation. Where, among friends in casual conversation, the word normal tends to refer to what is expected, the normal status of the switch is, explicitly, its contacts electrical status when no stimulus is applied, that is, when the switch is at rest. An applied example of this definition is a momentary-contact pushbutton switch is not being pressed, because, when the pushbutton is not being pressed, the switch is experiencing no physical stimulation. Electrical schematic drawings always represent switches in their normal status. When an electrical switch on a lamp is in its normally-open state, the switch is open while receiving no physical stimulation.

The concept of normal is somewhat more complex when applied to pressure and temperature switches. Pressure and temperature switches are actuated, not by electrical signal or human contact, but by process related stimuli, i.e. temperature, flow, pressure, or level. A flow switch is actuated by a defined amount of flow through a pipe. Lets say a flow switch is engineered to trigger an alarm when the flow rate inside a pipe is below a certain level. Even if the contacts of the flow switch are designated as being in their normally-closed status, the switch will be open when enough fluid is flowing through the pipe. The normal switch status (closed) indicates an abnormal process flow rate condition, because the switch is only going to be in its normal electrical status when the flow is low. Considering this inverse nature (normal switch status indicating abnormal process status), switch contacts are conventionally represented in accordance with the switch operation and not the process operation. The manufacturers of the pressure and temperature switches cannot predict the normal status of particular processes in which their switches will be used. By utilizing the conventional switch terminology, there is a common status designation for the normal status of the switch. The designation is applicable and readable regardless of the process conditions of the specific industry using the switch. This convention provides for universal comprehension of control system electrical schematics and other symbolic representations of control system operation.

Pressure switch (courtesy ofUnited Electric Controls)

In making the connection between the normal state of switch contacts and the normal state of a process, one should relate the switch state to the process condition which would serve as the stimulus to change the switch state. For a limit switch, which responds to physical contact by an object, normal means the target is not contacting the switch. For a proximity switch, normal means the target is far away. A normal pressure switch condition occurs when the pressure is low, or may even indicate a vacuum. Level switches are normal when the level is empty. Normal for a temperature switch means the temperature is low. Flow switches are normal when there is a low flow rate, or the fluid is stopped. Both an understanding of normal as defined by the manufacturer of the switch and normal in terms of industry specific processes is necessary to correctly interpret the status of an operation. Once the concept of normal used in everyday conversation is uncoupled from your process control thinking, things fall into place easily.

Tuesday, December 15, 2015

Most industrial applications require the monitoring of pressure and temperature of a process. Pressure and temperature measurement can be accomplished either by transmitters, gauges or by switches.
This post will provide a quick introduction of industrial electromechanical pressure switches and temperature switches.

An industrial pressure and temperature switch is made up of the three main components: 1) the sensor, 2) the housing and 3) the switching element.

The correct combination of each component assures proper application of the device for its intended use.

Sensor

The sensor is located above the pressure port and process connection. For pressure and differential pressure switches, there are several varieties of pressure sensors to choose. The most common types of pressure sensors are:

Metal Bellows - an accordion-like device that provides linear expansion and contraction based upon the application of pressure or vacuum. Bellows are excellent sensors because they provide good overall pressure range and are fairly sensitive to small changes in pressure.

Piston - A rod and o-ring combination that moves linearly in direct response to applied pressure. Piston sensors are normally only applied to only very high pressure ranges. They have very small surface areas and wide deadbands (the change in pressure required to change the position of the switch output).

Diaphragm - A thin, elastomer or metallic membrane, often with a rolled lip that allows for greater movement. The diaphragm has a large surface area and provides the most sensitivity to pressure change, making it ideal for low to mid-range pressure sensing.

Housing

Housings are classified and selected based on the atmosphere in which they’ll be used. Housing ratings are classified by several national and international agencies such as NEMA and CENELEC. Very generally put, housings can be rated as general purpose, dust & water resistant, water tight, corrosion resistant and hazardous (explosive) environments. Proper selection of the housing is important to the operation and life expectancy of the device. In hazardous environments, proper selection is absolutely critical. If unsure about the housing classification, consultation with an applications expert is required.

Switching Element

The switching element refers to the signaling device inside the enclosure that responds to the movement of the sensor. It can be either electrical or pneumatic, and provides an on-off signal (as opposed to an analog, or proportional signal produced by transmitters).

The switching element is most times a “micro” type single pole, double throw (SPDT) electrical switch. These microswitches come in many configurations and electrical ratings, such as double pole, double throw (DPDT), 120/240 VAC, 12VDC, 24VDC, and hermetically sealed.

For the switching element and the sensor, it is very important to know the cycling rate (number of on vs. off times over a period of time) the instrument will see. Since both of these elements are mechanical, they will eventually wear out and need to be replaced. Switches are an economical and strong performing choice for low to medium cycle rates. For extremely high cycle rates, the use of solid state transmitters are a better choice.

An electromechanical temperature switch (sometimes called a thermostat) is, for the most part, a piston type pressure switch connected to an oil filled capillary and bulb sensing element. The thermal expansion of the oil inside the bulb and capillary creates the pressure and linear movement upon the piston sensor of the switch. The bulb and capillary elements can be supplied in copper or stainless steel, and at various lengths.

There are many more details to selecting and applying electromechanical pressure and temperature switches. This post is only intended to provide a very general introduction. It is always suggested to discuss your application with a qualified applications engineer so that you are assured to get the longest lasting, most economical and safest instrument possible.

Thursday, October 29, 2015

Sadly, There are too many recent examples of catastrophic industrial accidents. New safety technologies exist today that can prevent or mitigate future disasters. The philosophy of safety is changing - the focus on plant safety has changed from reactive to a proactive approach. End users have a new sense of urgency toward safety processes.

The United Electric Controls (UE) Series One is a SIL-certified (SIL stands for safety integrity level) transmitter designed solely for safety, alarm, and shutdown applications, with reliability, speed, and fewer nuisance trips. It is also designed for both greenfield and brownfield installations, and is cyber secure.

A typical safety loop consists of sensors (such as a pressure transmitter), controllers, and final control elements. Most SIL-rated pressure transmitters require 300ms to communicate with the controller and up to 500ms for the controller to send a signal to the final control element (such as a valve). This may not be fast enough for critical applications. By connecting the One Series Safety Transmitter directly connected to the final control element, the signal speed is reduced to 100ms - a huge time savings when you're in the midst of a disaster. When used with blowers, pumps and compressors, the One Series makes up a complete safety system with a self-contained sensor, controller, and final control element (the switch) capable of SIL2 without additional safety instrumented function (SIF) components.

The below document provides detailed information about the Series One.

Thursday, September 17, 2015

Safety implementation typically is done by a group that includes plant instrument engineers and technicians, who are charged with finding simple and reliable solutions. Often, these situations involve the question of when to shut a process down. Such decisions frequently hinge on key process variables such as flow, level, temperature and pressure. these must be in a specified range at various locations within chemical and petrochemical plants, refineries and power plants, including everything from critical process vessels to eye wash stations.

For such point safety applications, a properly designed and implemented digital switch with self-diagnostics can be an important part of the answer. As an element of a multiple technology solution, a digital switch-based approach can help eliminate common-mode failures, significantly improve response time, achieve needed safety integrity levels (SILs), and simplify plant safety instrumentation.